The present invention relates to a semiconductor integrated circuit, and in particular to an inverter circuit including a three-phase bridge for converting direct current into alternating current.
Heretofore, a three-phase bridge inverter circuit is used for controlling the rotation speed of a brushless motor, an induction motor, etc. A prior art inverter circuit used for these uses was composed of output semiconductor switching elements and drive circuits therefor combined with parts for individual elements.
As a prior art technique, by which such an inverter circuit is modularized in a small size, there is known a technique discribed e.g. in "GESmart MODULE SIMPLIFIES MOTOR DRIVE DESIGN", GE Application Note Pub. #200.96, Nov., 1985.
By this prior art technique, a half bridge, in which two switching elements corresponding to one phase of an inverter are connected in series with each other, is accommodated in one module together with drive circuits for the respective elements, in order to reduce the size and improve the usability.
By the prior art technique described above, as a measure for reducing the volume of an inverter circuit, a part corresponding to one phase of an inverter is modularized. Three modules are necessary for driving a brushless motor, an induction motor, etc. with variable speed, using modules according to the prior art technique described above.
For this reason, the prior art technique described above cannot deal with the requirement to integrate inverters in a chassis within a motor.
In order to solve such a problem, it is desired to form inverters in the form of a monolithic IC capable of reducing remarkably the volume thereof by integrating parts corresponding to three phases in an IC altogether. However, in order to obtain a voltage corresponding to the commercial voltage, which is 100 V, a withstand or breakdown voltage of 250 V between different switching elements within the IC is required.
In the case where an inverter circuit having such a withstand or breakdown voltage between different elements is composed of an IC fabricated by a process of the prior art pn junction isolation method, a problem arises that the chip area is increased significantly for the purpose of increasing the withstand or breakdown voltage between different elements. Further, because of the fact that the potential at an output is varied in a pulse-like manner by on-off operation of an output switching element in a bridge inverter, noise current flows through parasitic capacitance of another output switching element and another problem arises in that erroneous operations of the circuit take place in this way.